Vertical-cavity surface-emitting lasers (VCSELs) are widely used in today's high-speed optical data communications channels. One problem, however, with the use of VCSELs is their electrical and optical power characteristics, which can vary a great deal across process and temperature variations. This imposes limits on the driver to operate at high data rates.
On the electrical side, the bandwidth of the driver to the first order is proportional to the RC time constant of the driver, where R is the VCSEL termination equivalent resistance and C is the total load capacitance seen by the driver and the VCSEL combined. The driver's bandwidth is mainly limited by the driver's load capacitance and large variations in the VCSEL resistance and capacitance. This results in large variations in the bandwidth and, thus, data transition behaviors having either excessive peaking or significant Inter-Symbol-Interference (ISI).
On the optical side, however, due to the optical power and extinction ratio (P1/P0) compliance requirements among other specifications, a great deal of programmability in VCSEL output power must be introduced to its biasing structure to handle such process and temperature variations. This could force the driver to accumulate even larger load capacitance and, thus, limiting the bandwidth of operation.
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